The invention relates to surface acoustic wave device substrate members, and in particular to improvements in the piezoelectric coupling and diffraction loss characteristics of such devices.
Surface acoustic wave devices such as filters, delay lines, encoders, decoders, correlators and other signal processing means commonly use a standard ST cut quartz substrate. This substrate, however, has a typically low piezoelectric coupling factor. A significant problem exists then when there are required low insertion loss, temperature compensated surface acoustic wave devices having larger bandwidth than those obtainable in devices built on ST cut quartz.
Currently, lithium niobate (LiNbO.sub.3) is used in surface acoustic wave devices requiring greater bandwidth (for a given amount of insertion loss) than that obtainable with ST cut quartz. However, since LiNbO.sub.3 is not temperature compensated, bulky and costly overns are required for temperature control.
Other attempts to solve the problem have met with various degrees of success. TI.sub.3 VS.sub.4, for example, has been found to be temperature compensated with substantially better piezoelectric coupling than ST cut quartz. The SAW velocity for this material, about 900 m/sec, is relatively slow, which is a disadvantage for high frequency filter applications but an advantage for long delay lines. The main drawback to the material is that the electromechanical power flow angle corresponding to the temperature compensated cut is rather large, about -17 degrees.
Another substrate material, a temperature compensated composite, produced by sputtering a film of silicon dioxide on YZ lithium tantalate, has been found to exhibit a very small electromechanical power flow angle, a piezoelectric coupling of about 0.007, and a relatively large SAW velocity. The most attractive feature of the material is that its second order temperature coefficient of time delay is nearly an order of magnitude smaller than that of ST cut quartz. The main drawbacks of the composite are: (1) the need to very accurately control the thickness of the S10.sub.2 film; (2) the loss in the film at high frequencies, and (3) the SAW dispersion in the film.
Other state of the art solutions are subject to the same or similar trade-offs and limitations.
A high piezoelectric coupling temperature compensated SAW substrate has been discovered and disclosed in U.S. Patent Application of Robert M. O'Connell, entitled "HIGH PIEZOELECTRIC COUPLING TEMPERATURE COMPENSATED BERLINITE SUBSTRATE MEMBER FOR SURFACE ACOUSTIC WAVE DEVICES," filed on even date herewith. Although the device disclosed therein is an improvement over prior art devices and meets the temperature compensation and piezoelectric coupling requirements, it exhibits diffraction losses that exceed those of ST cut quartz. There currently exists the need therefore for a surface acoustic wave substrate that has improved diffraction loss characteristics as well as temperature compensation and improved piezoelectric coupling. The present invention is directed toward satisfying that need.